This invention relates to insulating compositions. More particularly, this invention relates to insulating compositions useful for making insulated, smoke and fire resistant plenum communications cables typically used at frequencies ranging from DC up to several hundred MHZ.
Fluorocarbon polymers have been used as insulation for various purposes including plenum-type communications cables. The insulated conductors may be used alone, or as in a coaxial cable construction, or twisted with other conductors as pairs which may be combined to form a multiple pair core. The core may be coated with a jacket consisting of a fluorocarbon polymer or other flame retardant, low smoke polymer that will pass applicable regulatory requirements for plenum rated communications cables. Because of the possibility of fire and subsequent spreading of smoke within a building through the air plenum, plenum approved fire retardant, low smoke cables must be used for those applications.
Fluorocarbon polymer materials are often used as insulations because of their fire retardant properties and low smoke emissions when ignited. The electrical properties of fluorocarbon polymers are typically not as good as polyolefins, such as polyethylene or polypropylene and are more costly. The most pertinent electrical properties include the dielectric constant and dissipation factor. Dissipation factor in particular is important for cables designed to operate at frequencies in excess of 10 MHZ. The higher the dissipation factor of an insulation material the more a signal is attenuated as it is transmitted through a transmission line, such as a communications cable. The effect of dissipation factor becomes greater as the transmission frequency is increased. Fluorinated ethylene-propylene (FEP) resin is a melt processible fluoropolymer that has both good electrical properties and adequate flame and smoke performance. Due to the high cost and limited availability of FEP; alternative materials are desirable.
To lower the dielectric constant and improve the dissipation factor a foamed insulation of the polymer coating over the wire may be used. An example of such process is taught in U.S. Pat. Nos. 4,472,595 and 3,643,007 for polyolefin insulations.
An example of efforts to use fluorocarbons in cable insulation is described in co-pending application Ser. No. 65,860, filed May 21, 1993, now U.S. Pat. No. 5,462,803, which provides an insulating layer of a copolymer of ethylene and chlorotrifluoroethylene (commercially available under the trade name HALAR.RTM. copolymer and an outer layer of polyvinyl chloride.
In another example, foamed electrical insulation of ETFE and ECTFE is described in U.S. Pat. Nos. 4,331,619 and 4,394,460 wherein the process uses a chemical blowing agent and a nucleating agent. Polyethylene is added in small amounts as a processing aid. These patents do not address electrical attenuation performance, which is not adequate to meet some current 100 MHZ twisted pair applications. These patents also do not address potential degradations of the insulations mechanical properties as a result of this foaming process. Insulation elongation performance in particular is negatively impacted by foaming.
Since these insulating compositions are used in environmental conditions from about 0.degree. C. to 60.degree. C., the effect of temperature on electrical properties is important and specified in application standards, such as TIA/EIA 568A. Attenuation, in particular, is affected negatively by increases in temperature. ETFE and ECTFE insulated cable exhibit increases of approximately 0.6% per degree centigrade temperature increase at a frequency of 100 MHZ. This is more than twice the 0.25% increase measured when using a polyolefin or FEP insulation. While one may design a cable utilizing a foamed ETFE or ECTFE insulation that meets room temperature requirements for high frequency applications, a product that will also meet the applicable elevated temperature standards will be physically larger than desirable.
There is need for a melt processible insulation material with good physical properties that may be used as a substitute for FEP in plenum cable applications that require high performance signal transmission at frequencies in excess of 10 MHZ.